1. Field of the Invention
The present invention relates generally to the field of sheet feeding apparatus, and, more particularly, to a vacuum feeding apparatus for feeding printing plates one-at-a-time from the bottom of a stack of plates to a printing press.
2. Description of the Prior Art
Bottom feeding from a vertical stack of sheets by means of a movable suction device is well known in the prior art (see, for example, U.S. Pat. No. 3,934,868 to Selak and the patents cited therein). Typically, these feeding systems employ a suction cup coupled to a suitable vacuum source and moveable into contact with the bottom face of the lowermost sheet in the stack for the purpose of attaching itself to the sheet, pulling it downwardly and then depositing it on a transfer means for delivery to a utilization device.
Although suction systems such as these are generally effective in feeding sheets one-at-a-time from the bottom of a stack, they do suffer from one particularly significant deficiency. Specifically, for them to operate effectively it is necessary that the sheet to be picked up lie quite flat so that the rim of the suction cup can properly seal against it so as to enable an adequate suction to be established and maintained to hold the sheet and carry it from the bottom of the stack to the transfer means. If the sheet is even slightly curled or arched or otherwise deformed, the suction cup will not be able to properly seal against it and jamming will occur necessitating operator intervention.
In multiple-stage automated systems employing the sheet feeder, for example, in an automated printing system, reliability is obviously very important. Accordingly, a sheet feeding system which will operate with improved reliability and with a minimum need for operator intervention would be a significant advance in the art.